Issue 50

A.G. Lekatou et alii, Frattura ed Integrità Strutturale, 50 (2019) 423-437; DOI: 10.3221/IGF-ESIS.50.36 430 Higher magnifications in Fig.5 manifest the positive effect of FA addition on the corrosion resistance of the 304L rebars in the low pH electrolyte. Fig.5 shows more or less compact films uniformly deposited on the surface of 304L, in the cases of the 20 wt.% FA (Figs.5(a-d)) and 25 wt.% FA (Figs.5(e-g)). These films are probably products of interaction between Ca(OH) 2 -FA. The interaction is drawn from the detection of (besides Ca) Si, Al, S and occasionally Mg on them. The de- tection of noticeable amounts of Ca, Si, Al, S, Mg, O and Cl in the EDX spectrum of Fig.5b constitutes evidence that pozzo- lanic type reactions have occurred between Ca(OH) 2 , H 2 O, FA and probably AR, leading to the formation of complex compositions, as reported in Introduction. Also, the detection of Cl - raises the possibility of salt formation analogous to the Friedel’s salt. The detection of Cr and Fe (Fig.5b) indicates interaction between FA, Ca(OH) 2 and steel. Figure 4 : Cross-sections of 304L rebars after cyclic potentiodynamic polarization in acidic electrolyte containing Ca(OH) 2 , acid rain simulating solution and (a) 0 wt.% fly ash, (b) 20 wt.% fly ash and (c) 25 wt.% fly ash (wt.%: with respect to (Ca(OH) 2 + fly ash)). However, Fig.6 demonstrates that FA may also have a destructive effect on the corrosion resistance of the 304L rebars at high FA amounts. Acicular pits appear to be associated with high FA additions, in the following manner: Agglomerates of FA do not have a vigorous pozzolanic activity with Ca(OH) 2 due to their low specific surface area as compared to the fine particles. As such, they will not form a uniformly deposited C-S-H film on the surface of the steel, which is resistant to ag- gressive ion penetration. Instead, the agglomerates can locally form thick deposits on the steel surface. Consequently, dif- ferential aeration cells may be formed, where the steel below a relatively compact part of the deposit (reduced pO 2 ) acts as an anode and the steel below a less dense part of the deposit (higher pO 2 ) acts as a cathode. The anode corrodes forming pits filled with corrosion products (from FA-Ca(OH) 2 -AR-steel interactions). A complementary reason for the localized corrosion observed at high FA amounts in the low pH electrolyte, can be the extensive formation of hydrated sulphates by the interaction of FA-Ca(OH) 2 -AR, as mentioned in Introduction. When this formation takes place in a preexisting defect of the steel surface, the resulting volume expansion will exert stresses to the defect and eventually stress corrosion cracking. This reason can justify the formation of acicular pits with short but needle-like branches, as well as for the detection of high concentrations of sulfur inside the pits (as shown in Fig.6b).